Bandwidth, that unavoidable but necessary range of frequencies that must be made available to transmit message signals has, in recent years, become the target of intense efforts by scientists and engineers working in the telecommunication field. These efforts are directed towards discovering techniques to reduce the bandwidth to the smallest range possible, without hindering the rate at which the message information can flow. In effect, what is being sought are signal processing methods that optimize the required bandwidth consistent with the information rate of the message that must flow through the channel.
Message signals, such as speech, music or television have inherent bandwidths that are greatly in excess of that which is theoretically necessary for the actual information transfer rate contained in these types of signals. The burdens associated with this excessive bandwidth are tolerated because no effective, practical signal processing methods have heretofore been discovered that can encode these signals, in real time, so as to permit the use of a smaller range of frequencies. This encoding must, of course, be coupled with a corresponding decoding process so that, after transmission, reconstruction of the encoded signal yields the original message signal. The lack of any really practical bandwidth reduction scheme means that there is a tremendous overcrowding of the radio spectrum. For example, television broadcasting must be aired on VHF and UHF and even microwave communication is rapidly becoming difficult due to a lack of clear channels. In fact, the communications industry has been forced to go to optical methods just to accommodate the vast need for bandwidth to meet today's needs for telecommunication services.